A Phase II randomized, open-label, 4-arm trial comparing dual and single protease inhibitor salvage antiretroviral therapies for HIV infected subjects who have virologic evidence of nelfinavir treatment failure and who are naive to non-nucleoside reverse transcriptase inhibitors and to both components of at least one of the following nucleoside reverse transcriptase inhibitors combinations: 1) ZDV and 3TC, d4T and 3TC, d4T and ddI or 4) ZDV and ddI. Eligible subjects will be randomized in an open-label fasion to the 4 treatment arms. Plasma HIV RNA will be quantified in real time and the study monitored closely for early virologic endpoints. An interim analysis will be performed after 40 study endpoints have occurred to identify underperforming or poorly tolerated arms. Interim analyses will be repeated after each additional 40 endpoints.